The present invention relates to a device for mounting a gripper to a working tool mounted on a sliding carriage of a piercing machine. The gripper is of the type which couples a rod for piercing the taphole of a shaft furnace. The gripper has a female thread designed to be screwed onto a threaded end piece of a working tool.
A gripper of the type described above is disclosed in French patent document No. FR-8301046. The gripper is particularly intended for machines which carry out a process in which the closing and opening of the taphole involve an operation of installing and an operation of extracting a piercing rod. The piercing rod is discarded in the mass of the taphole between each successive casting.
These grippers function satisfactorily and are well suited for their intended purposes. However, since these grippers have been put into operation, numerous breaks have been found in the region of the threaded end piece of the working tool. There are many reasons for these breaks. One reason is that the gripper is subjected to many stresses, starting with the stresses arising as a result of the normal use of the gripper, especially under the action of the striker. The impacting of the striker tends to loosen the gripper from the working tool, and an excessive play in the thread accelerates the wear and destruction of the striker. Furthermore, the end piece of the working tool is constantly subjected to the bending moments arising as a result of the dead weight of the gripper which can be on the order of 50 kilograms.
In most cases, the breakage and wear of the thread are caused by stresses which arise from abnormal use which are superimposed on the existing stresses described above. For example, because of the dead weight of the gripper, it is very difficult to properly screw the gripper onto the threaded end piece of the working tool. This operation, which is carried out manually, involves alignment of the gripper with the threaded end piece and rotating the threaded end piece until it is engaged with the corresponding female thread of the gripper. At this moment, a pneumatic motor is usually activated to cause the end piece to rotate to ensure that the gripper is automatically clamped onto the end piece. If the gripper and the end piece are not properly threaded when the pneumatic motor is engaged, the threads will strip.
Another problem that often arises is the retraction of the machine from its working position when the tip of the piercing rod is not completely released from the mass of the taphole. The purpose of this retraction step is to save the machine from splashes of molten metal which occur as soon as the taphole is opened. In this instance, the piercing rod is exposed to very large bending moments which considerably increase the risk of breakage of the threaded endpiece of the working tool.